Hypothesis

Age-related decline in mitochondrial function within gut enteroendocrine cells (EECs) shifts tryptophan metabolism toward the kynurenine pathway and reduces serotonin synthesis, producing circulating metabolites that prime microglia and sustain neuroinflammation independent of microbiota composition.

Mechanistic Rationale

• EECs are the primary source of gut-derived serotonin (~90% of body total) and express tryptophan hydroxylase 1 (TPH1). Mitochondrial ROS generated with age impairs TPH1 activity and diverts tryptophan to indoleamine 2,3-dioxygenase (IDO1), elevating kynurenine and quinolinic acid [1].
• These metabolites cross the compromised gut barrier (see [2]) and reach the brain, where quinolinic acid acts as an NMDA agonist and kynurenine activates aryl hydrocarbon receptor on microglia, promoting a pro-inflammatory phenotype [3].
• Simultaneously, reduced serotonergic signaling diminishes vagal anti‑inflammatory tone, further loosening the gut‑brain barrier and creating a feed‑forward loop.

Testable Predictions

1. In aged mice, pharmacological rescue of mitochondrial function in EECs (e.g., with the mitochondria‑targeted peptide SS‑31) will restore TPH1 activity, lower serum kynurenine/quinolinic acid, and increase gut‑derived serotonin without altering fecal microbiota composition.
2. Such treatment will decrease microglial Iba1‑positive activation in the hippocampus and improve performance on spatial memory tasks (Morris water maze) to a degree comparable to young‑donor FMT.
3. Conversely, genetic inhibition of TPH1 specifically in EECs will abolish the protective effect of SS‑31, confirming that the benefit depends on EEC‑derived serotonin.

Experimental Design (falsifiable)

• Use 20‑month‑old C57BL/6 mice; randomize to: (a) vehicle, (b) SS‑31 (3 mg/kg i.p. thrice weekly), (c) SS‑31 + EEC‑specific TPH1 knockout (Vil‑Cre; Tph1^fl/fl), (d) young‑donor FMT as positive control.
• After 4 weeks, collect feces for 16S rRNA sequencing to confirm microbiota unchanged across groups (except FMT).
• Measure serum tryptophan, kynurenine, quinolinic acid, and serotonin via LC‑MS/MS.
• Assess colonic barrier integrity (FITC‑dextran efflux) and hippocampal microglial activation (Iba1 immunohistochemistry, flow cytometry for CD68).
• Behavioral testing: novel object recognition and Morris water maze.

Expected Outcomes & Falsifiability

If the hypothesis is correct, SS‑31 will normalize tryptophan metabolites, reduce microglial activation, and rescue cognition despite an aged microbiome. Lack of metabolic or behavioral improvement, or improvement that persists despite EEC‑specific TPH1 loss, would falsify the claim that EEC mitochondrial dysfunction is a necessary driver of inflammaging via this serotonergic/kynurenine axis.

Broader Implications

This positions gut EEC mitochondria as an upstream, microbiota‑independent lever to break the gut‑brain inflammaging loop, complementing existing gerotherapeutic strategies such as rapamycin or senolytics that do not directly target neuroactive metabolite fluxes.

References

[1] https://www.frontiersin.org/journals/aging-neuroscience/articles/10.3389/fnagi.2025.1667448/full [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC11866832/ [3] https://doi.org/10.1101/2023.11.17.567594